ID of Brown Sponge?

[LancasterJohn]

This has recently started growing in my aquarium and I think it may be a sponge of some kind. It's brownish colored and has a velvety texture. It has started to take over the area where some of my polyps are. Is this indeed a sponge and is it harmful in any way? Any help is appreciated.

 

[billsreef]

It looks like some kind of encrusting sponge, usually a good sign that they are growing so well